Doping in amplifying devices has traditionally been centered within the fiber core, in order to achieve a good signal. It has recently been suggested that optical waveguide amplifiers and lasers be fabricated with the dopant placed in an annular distribution about the core, i.e., with the dopant distributed between the center and the core/cladding interface. An optical amplifier with this type of dopant distribution is shown in U.S. Pat. No. 5,259,046 (issued to DI GIOVANNI et al on Nov. 2, 1993), entitled "Article Comprising an Optical Waveguide Containing a Fluorescent Dopant". The object of this patented apparatus was to provide an optical amplifier that was relatively insensitive to variations in cut-off wavelength, as well as pump-power modal distribution. This insensitivity provided a means of fabricating optical fiber having reduced tolerances.
The aforementioned patent, however, discloses a problem with this annular, doping technique, in that the amplifier gain drops with a decrease in the wavelength. Thus, amplifiers which employ an annular, doping configuration have a higher threshold power, but it is at the expense of lower gain. However, amplifiers having a low gain are generally ineffective.
The above, annular, doping configuration can also be used for a different purpose, one in which the lower gain limitation becomes irrelevant. The current invention advances the idea that doping the cladding of an optical fiber can provide an optical device which can be used as a final stage, either a pulse- or continuous-wave amplifier, or a Q-switch, with each having a high-value saturation output.
In the past, amplifiers configured to provide high-output power had an inability to store energy as an inverted population, because the amplified, spontaneous emission in the doped core depleted the inversion. The present invention, however, reflects the discovery that an annular, doping technique makes possible an optical fiber having a pumped energy that will be insensitive to depletion inversion. In such a situation, a low gain is not important. What is important is that one can construct a high-output power amplifier, or a Q-switch.
This invention suggests a single-mode fiber design with active, rare-earth ions (such as neodymium Nd!, erbium Er!, thulium Tm!, praseodymium Pr! and ytterbium Yb!) located in an annular region within or next to the cladding, i.e., straddling the core/cladding interface. The core is substantially free of any gain material, and the index of refraction is essentially the same for both core and cladding. The transition probability of induced emission is proportional to the square of the transverse component of the field. Therefore, the active ions in the annular region have a diminished tendency for induced emission, resulting in the ability of the inventive fiber configuration to store more inverted ions without being depleted by amplified, spontaneous emission.
In one embodiment of this invention, the fiber has indices of refraction between the core and the active ion interface that are essentially the same. A transverse component of the electrical-field strength in the core of a single-mode, step-index fiber is defined by a Zero.sup.th order Bessel function J.sub.o. The field strength in the cladding is defined by a modified Hankel function K.sub.o. The fiber is designed with a "V" value for the modified Hankel function, which is less than 2.405, a value consistent with single-mode operation, as defined by J.sub.o. The radius of the core "a" will be less than 15 microns, and the optical fiber can operate with a wavelength of 1.5 microns.
Such a light-pumped fiber device provides an amplifier in which the induced emission per given amount of energy that is stored in the excited state population is relatively low. The central part of the core both stores and propagates whatever light is present, due to induced emission, which takes place only in the annular region. The light that is incident on the fiber is of sufficient intensity, so that, despite the low rate of induced emission per unit of light intensity, there is a significant power increase.